Drive control method and drive system operating according to said method

ABSTRACT

In a drive control method for a vertical mill having a grinding plate rotatable about the vertical axis and being drivable by a drive train that includes an electric motor and a gearbox, a rotational speed of the grinding plate is varied cyclically, especially intermittently. A corresponding drive system operating according to the method is also disclosed.

The present invention relates to a drive control method, namely a methodfor controlling a heavy duty drive, in particular a heavy duty drive fora vertical roller mill for comminuting brittle materials such as cementraw material, and to a corresponding drive system operating according tosaid method.

Vertical roller mills of the above mentioned type, having a grindingtable rotating about the vertical and grinding rollers above thegrinding table, are subject to severe mechanical vibrations. Theresulting forces and torques can be so powerful that the grindingprocess has to be stopped in order to prevent damage to the drive train,namely the electric motor and gearbox in particular, or to the plant asa whole.

In order to minimize such vibrations, the mill operator has hitherto hadto design the process parameters, i.e. in particular the contactpressure of the grinding rollers, composition of the material to beground and amounts of grinding additives such that the vibrationsexcited remain below a critical level. However, this means undesirableprocess design limitations which negatively impact many areas. Theseinclude the range of products that can be made from the respectiveground material obtained, the effectiveness of the mill, the energyinput required and the cost-efficiency. Nevertheless, such measures areunreliable, as much experience is required for correct process controland the properties of the natural materials before and after grindingare necessarily always different. This makes it necessary tocontinuously optimize the process and adjust it to the raw material.

However, extreme vibration states—known in the industry as “rumbling” ofthe mill, repeatedly occur, with the result that the mill has to bestopped and then restarted. This is to the detriment of the availabilityand productivity of the plant. There is also the risk of gearbox andplant damage. To obviate this problem, process control has hitherto beenorganized particularly defensively in order to prevent these millvibrations as far as possible. However, the production rate, productquality and the range of manufacturable products suffer as a result.

Against this background and because of the increasingly exactingrequirements in respect of availability, efficiency and Total Cost ofOwnership (TCO), the design and arrangement of the electrical andmechanical components of a drive system and of the respective drivetrain of a heavy duty drive, in particular of a vertical roller mill,are becoming increasingly important.

For vertical roller mills, drive systems comprising a gearbox and anelectric motor in the form of an asynchronous motor, preferably a woundrotor, and a frequency converter feeding the electric motor constitute apreferred solution. Here the mill gearboxes are in practice implementedas variants of bevel or spur gear planetary mechanisms. The purpose ofthe gearing arrangement is not only speed and torque conversion but alsoto absorb the axial grinding forces and transfer them into the base.

Controlling such a drive system for a vertical roller mill essentiallypresents the following problems in practice:

In order to be able to ensure optimum process control, the first,apparently trivial task of the drive is to deliver the predefinedrotation speed of the grinding table. As the process torque demand atthe grinding table fluctuates, speed control is required.

The load fluctuations and vibration excitations acting on the drivemechanism are influenced by impulsive loads such as those produced whenthe grinding rollers encounter coarse material, stochastic loads of thegrinding process, periodic excitations from the gearbox and millkinematics, and a varying contact pressure of the grinding rollers. Theinteraction of these load influences results in a complex load cyclewhich can even set off resonance vibrations.

In addition to the drive train vibrations, an unstable, i.e. fluidizingor undulating grinding bed, for example, can also cause extremevibrational states of the mill, in particular mill rumbling.

Lastly the grinding of natural products makes it largely unpredictablehow the grinding process must be adjusted in order to guarantee quietrunning of the mill. It is therefore always a challenge for the operatorat the control desk to find the correct process parameters. In the end,although the drive alone can quieten a poorly adjusted process, itcannot correct it.

The approach proposed here deals with the effect of undesirable rumblingof a mill as the result of a particular surface structure of thegrinding bed, and an object of the present invention is accordingly tospecify a means of efficiently preventing or at least reducing suchrumbling.

One reason for mill rumbling caused by the surface structure of thegrinding bed is that the mill, as a resonant system, reacts tostochastic excitations from the grinding process e.g. with regularrelative movements between the grinding rollers and the grinding bed.These regular relative movements result from the particular naturalfrequency of the mechanics and kinematics of the grinding rollers whichare disposed in a movable, in particular swiveling manner above thegrinding table and the grinding bed produced thereon during milloperation. The grinding rollers act on the grinding bed on the one handbecause of their own weight and because of their movable, in particularswivel mounting. In addition, the action of the grinding rollers on thegrinding bed can be intensified still further by an additionally appliedcontact pressure.

The above mentioned object is achieved by a method for drive control ofa vertical roller mill having the features as claimed in claim 1. Theobject is also achieved by a drive system having the features of theparallel device claim. The vertical roller mill, also referred to hereand in the following sometimes merely as the mill for short, comprises agrinding table rotating about the vertical, wherein the grinding tablecan be driven by means of a drive train comprising at least one electricmotor and usually a gearbox and is driven during operation of the mill.The method is characterized in that the rotation speed of the grindingtable is cyclically varied.

In the case of a drive system designed to carry out such a method andpossibly one or more of the embodiments described below, namely a drivesystem for a vertical roller mill comprising a grinding table rotatingabout the vertical, the drive system comprises at least one electricmotor, optionally a frequency converter feeding the electric motor, agearbox between the at least one electric motor and the grinding tableand optionally a sensor system for obtaining vibration-relevant measuredvalues, in particular vibration-relevant measured values in the form oftorque or speed measurements with respect to a rotation speed of arotating component of the vertical roller mill or to at least onedriving and/or supporting torque acting in or on the gearbox. The drivesystem is characterized by a speed variation device with which therotation speed of the grinding table can be varied, wherein the speedvariation device is designed and set up to operate according to themethod outlined above and described in greater detail below, and carriesout such a method during operation of the drive system.

In short, the invention is therefore a method and a device for drivecontrol of a heavy duty arrangement in the form of a drive system inwhich the cyclical variation of the rotation speed of the grinding tablehas the aim of creating no regular structure in the grinding bedsurface, i.e. no undulation of the grinding bed. Accordingly, thepurpose of the method and the device operating according to said methodis to prevent a possible cause of mill rumbling at source.

The advantage of the invention is that by cyclically varying therotation speed of the grinding table, rumbling of the mill can beprevented or eliminated or at least reduced without having to stop thegrinding process, and that this result is achieved by comparativelysimple intervention in the overall system, namely by appropriatelycontrolling the electric motor. The prevention, elimination or reductionof the vibrations will hereinafter be referred to as prevention forshort.

Cyclically varying the rotation speed of the grinding table is to beunderstood as meaning varying the rotation speed of the grinding tableusing an, in particular, variable speed profile in which thetime-averaged speed profile corresponds to the setpoint rotation speedof the grinding table. A special form of such cyclical variation of therotation speed of the grinding table is periodic variation of therotation speed of the grinding table, e.g. sinusoidally varying therotation speed of the grinding table about a setpoint rotation speed ofthe grinding table.

Advantageous embodiments of the invention are set forth in thesub-claims. References used indicate the further development of thesubject matter of the main claim by the features of the respectivesub-claim. They are not to be understood as a waiver of the achievementof independent, concrete protection for the feature combination of thesub-claims referred to. In addition, in respect of interpretation of theclaims in the case of a closer concretization of a feature in asubordinate claim it is assumed that such a limitation is not present inthe respective preceding claims. Lastly it is pointed out that themethod specified here can also be further developed according to thedependent device claims and vice versa.

In an embodiment of the method, cyclical or periodic variation of therotation speed of the grinding table is activated in a time-controlledmanner at predefined or predefinable, in particular equidistant pointsin time for a predetermined or predeterminable time period, e.g. everyfive minutes for ten seconds at a time. The cyclical or periodicvariation of the rotation speed of the grinding table is then notcontinuously active. Outside of the activation of the cyclical orperiodic speed variation, “normal” rotation speed of the grinding tableis produced.

In an alternative embodiment of the method, the cyclical or periodicvariation of the rotation speed of the grinding table is activated as areaction to a vibration-relevant measured value fluctuation exceeding apredefined or predefinable limit value.

Vibration-relevant measured values are all the measured values obtainedor obtainable in respect of the mill, the evaluation of which indicatesmechanical vibrations of the mill, in particular such mechanicalvibrations as are termed rumbling. Possible means of obtaining suchvibration-relevant measured values include sensors in the form ofvibration monitors, vibration sensors or the like which are e.g. mountedon the mill framework or other parts of the mill structure.Alternatively or additionally possible are also sensors which areassigned to the drive train where they acquire vibration-relevantmeasured values. For example, it can be provided that the power draw ofthe electric motor is measured by means of such a sensor andvibration-relevant data is derived from an electric motor power drawcorrelated with varying load situations so that in this respect measuredvalues relating to motor current drawn during operation of the mill ineach case are also an example of vibration-relevant measured values.

If such a measured value exceeds a predefined or predefinable limitvalue or possibly repeatedly exceeds such a limit value, in particularrepeatedly exceeds it within a predefined or predefinable period oftime, this indicates existing or impending mill rumbling. On the basisof such a detection or prediction of mill rumbling, cyclical or periodicvariation of the rotation speed of the grinding table is activated inorder to eliminate or at least reduce mill rumbling or prevent it fromoccurring in the first place.

In this variant of the method, the cyclical or periodic varying of therotation speed of the grinding table does not therefore take placecontinuously but only as required, namely if monitoring of therespective value obtained automatically indicates the necessity ofcounteracting an existing, undesirable vibration or risk of vibration inthe form of mill rumbling in each case.

In an embodiment of the method, cyclically varying the rotation speed ofthe grinding table takes the form of periodically varying the speed,wherein a frequency and/or an amplitude of the signal waveformunderlying the periodic variation of the rotation speed of the grindingtable and, alternatively or additionally, possibly also the signalwaveform itself is increased or reduced, i.e. changed, as a function ofa vibration-relevant measured value fluctuation exceeding a predefinedor predefinable threshold value. In this variant of the method, theperiodic variation of the rotation speed of the grinding table istherefore itself varied on the basis of automatic evaluation of theabove mentioned measured value. The threshold value in question can bebelow or above the limit value mentioned earlier. In the case ofcontinuous variation of the rotation speed of the grinding table, athreshold value below a limit value indicating existing or impendingrumbling is a possibility. The attainment or exceeding of such athreshold value then indicates that the already occurring periodicvariation of the rotation speed of the grinding table is insufficient toprevent or eliminate mill rumbling. In the case of periodic variation ofthe rotation speed of the grinding table solely as a function of amonitored limit value, the threshold value additionally considered inthis embodiment of the method will be above such a limit value.Exceeding of the threshold value is then an automatically evaluableindication that periodically varying the rotation speed of the grindingtable is insufficient for preventing or eliminating mill rumbling.

As a countermeasure, both in the case of grinding table speed variationthat is active continuously or only when required or intermittently, theperiodic variation is itself varied, e.g. by increasing the period ofthe underlying signal waveform. This is aimed at changing the undulationof the grinding bed and the aim of the thereby achieved change in theundulation of the grinding bed is in turn that no natural frequencies inthe overall mill system are excited by the resulting movements of thegrinding rollers because of grinding bed undulation and therefore millrumbling is prevented or eliminated, in any case at least reduced,namely e.g. in its frequency and/or vibration intensity and/or echospeed.

A sinusoidal profile, a triangular profile, a rectangular profile or aramp profile, for example, can be used for periodically varying therotation speed of the grinding table. Such profiles can easily begenerated using a signal generator or the like and, in the case ofsoftware implementation of the method, by appropriate mathematicalexpressions or families of characteristics.

In another or alternative embodiment of the method, a change in arespective signal waveform with which the rotation speed of the grindingtable is periodically varied is initiated as a function of avibration-relevant measured value fluctuation exceeding a predefined orpredefinable threshold value. This variant of the method ischaracterized by parallels with the changing (frequency, amplitude orsignal waveform) of the periodic variation of the rotation speed of thegrinding table already described above. Here, however, it is not theperiod of the periodic variation of the rotation speed of the grindingtable that is changed as a function of the exceeding of the thresholdvalue, but the signal waveform underlying said periodic variation. Thestatements made above may be referred to regarding the position of thethreshold value and for the use of this variant of the method either forcontinuous or merely as-required or intermittent variation of therotation speed of the grinding table. As a result, this variant isdesigned to change the grinding table speed variation according to theautomatically detectable exceeding of the threshold value whichindicates that mill rumbling cannot be prevented or eliminated using theexisting periodic variation of the rotation speed of the grinding table.This change is accomplished by changing the signal waveform. This alsoproduces a change in the undulation of the grinding bed. As in the caseof the method variant described above, this is designed to ensure thatno natural frequencies are excited in the overall mill system due to theresulting movements of the grinding rollers because of the grinding bedundulation, so that in this way the mill rumbling is prevented oreliminated, in any case at least reduced.

The two variants for changing the periodic variation of the rotationspeed of the grinding table, namely changing the period and changing thesignal waveform, in particular quantitative and/or qualitative changingof the signal waveform, are self-evidently also combinable.

Because it is particularly a matter of preventing a grinding bed surfaceregularity that is conducive to resonances, another possibility is toapply the individual methods in a quasi-random manner. A softwareimplementation of such a method provides all the possible variants ofthe method as functional units or the changing of the period and thechanging of the signal waveform as parameterizable functional units.Based on a random number generator or the like, individual functionalunits and/or parameters for the parameterization of the functional unitsor combinations of possibly individually parameterized functional unitsare then selected in order to counteract existing or impending millrumbling detected on the basis of the measured value monitoring outlinedabove.

In a specific embodiment of individual method variants described above,torque or rotation speed measurements are used as vibration-relevantmeasured values and are acquired. The further description of theapproach proposed here will be based, without loss of generality, onsuch torque or speed measurements. These will now be subsumed under theterm measured values. To obtain such measured values, a sensor system,i.e. at least one sensor incorporated in the sensor system or belongingto the sensor system, is used to measure a rotation speed of a rotatingcomponent of the drive train and/or at least one driving and/orsupporting torque acting on the gearbox.

A corresponding embodiment of the drive system outlined above ischaracterized by such a sensor system for obtaining a vibration-relevantmeasured value in the form of a torque or speed measurement, wherein bymeans of the sensor system a rotation speed of a rotating component ofthe vertical roller mill and/or at least one driving and/or supportingtorque acting in or on the gearbox can be measured and is measuredduring operation.

In this connection it should be noted that the use of sensors, e.g.sensors for acquiring torque or rotation speed measurements in the drivetrain (drive sensor system) instead of vibration sensors on the millstructure has apparently not been taken into consideration hitherto.However, according to the inventor's insight, mechanical vibration ofthe mill and therefore also rumbling of the mill can also be detectedusing such measurements. It is actually even the case that such measuredvalues replicate respective process events even more directly, as theevents in the grinding mechanism become much more clearly apparent,according to the inventor's insight, in the degree of rotational freedomof the drive than in the vibration level of the mill structure as awhole. This is because the rumbling is a periodic collapse of thesupporting effect of the grinding bed on the grinding rollers.Associated with the loss of this supporting effect, e.g. because ofundulation of the grinding bed or yielding aside of a fluidized materialto be ground, there also arises a breakdown of the loading torqueexerted by the grinding bed on the grinding table therefore directly onthe drive train. This torque characteristic is directly detectable insaid measured values, namely even before the grinding rollers move sostrongly that the vibration also perceptibly spreads to the rest of themill structure. Undesirable states such as mill rumbling or incipientmill rumbling can be detected earlier and more precisely in this way.Comparative studies have shown that detection of mill rumbling viavibration sensors usually takes place after a few seconds at theearliest. In this time the drive is subject to just over a hundred loadcycles (at a rumble frequency of e.g. 15 Hz). Evaluation of the drivesensor system allows rumbling to be identified after just three to tenload cycles, i.e. in less than one second. The advantage of using asensor system of this kind and of the thereby obtained measurements istherefore that, because mill rumbling can be detected earlier,countermeasures, i.e. even emergency shutdown of the drive (“emergencystop”), for example, but self-evidently also the cyclical or periodicvarying of the rotation speed of the grinding table highlighted here,can be initiated earlier and therefore the stressing of the drive, butalso of the mill as a whole, by mill rumbling can be reduced.

In one embodiment of the method, the electric motor is fed by afrequency converter and the rotation speed of the grinding table iscyclically or periodically varied by means of appropriate control of thefrequency converter. By means of a frequency converter, the describedcyclical or periodic varying of the rotation speed of the grinding tableby appropriate control of the electric motor, but also the changing ofthe period or the changing of the underlying signal waveform andcombinations thereof, can be comparatively easily achieved. Analternative to a frequency converter is a superposition gear with whichthe above described cyclical or periodic varying of the rotation speedof the grinding table can be achieved in basically an equivalent manner.

The method and the drive system operating according to said method arebased on the cyclical or periodic variation of the rotation speed of thegrinding table and on a speed variation device designed for thatpurpose. Individual aspects of the functionality of the speed variationdevice have already been described above. The functionality of the speedvariation device, the optional acquisition and processing of themeasured values in question functionally upstream of the cyclical orperiodic variation of the speed, and the resolution of the variation ofthe rotation speed of the grinding table functionally downstream of thespeed variation device can be realized in hardware and/or software. Inthe case of software realization, the invention is also a computerprogram having program coding means for executing all the steps of themethod described here and in the following when the computer program isrun on a controller or the like for a drive system for a vertical rollermill. Further, the invention is therefore also a digital storage mediumhaving electronically readable control signals which can interact with aprogrammable controller for a drive system for a vertical roller millsuch that such a method can be executed. Finally, the invention is alsoa drive system of the above mentioned type which comprises a processingunit and a memory, wherein such a computer program is loaded into thememory and is executed during operation of the drive system by theprocessing unit thereof.

An exemplary embodiment of the invention will now be explained ingreater detail with reference to the accompanying drawings. Equivalentitems or elements are provided with the same reference characters in thefigures.

It should also be pointed out that the approach described here andindividual and possibly combined embodiments can also be combined withthe approach proposed and specific embodiments described in the sameapplicant's parallel application attributable to the same inventor andhaving the applicant's internal reference number 201312099 (officialapplication number not yet known). In this respect, the completedisclosure content of this parallel application, especially havingregard to the therein described pattern recognition and the action onthe basis of a detected pattern, is implied in the description presentedhere.

The exemplary embodiment should not be interpreted as a limitation ofthe invention. On the contrary, within the scope of the presentdisclosure, changes and modifications are possible, especially suchmodifications and combinations that, for example, as a result ofcombinations or modifications of individual features or elements ormethod steps contained in the general description, in the descriptionsof various embodiments, and in the claims, and illustrated in thedrawings, can be comprehended by persons skilled in the art as far asthe achievement of the object is concerned and, as a result ofcombinable features, lead to a novel subject matter or to novel methodsteps and/or sequences of method steps.

FIG. 1 shows a greatly simplified schematic representation of a verticalroller mill comprising a grinding table driven by means of a heavy dutydrive,

FIG. 2 shows a plan view onto the grinding table and grinding bed,

FIG. 3 shows a periodically varied rotation speed of the grinding tableof the vertical roller mill, and

FIG. 4 shows a drive system of the vertical roller mill incorporating acontroller which causes the rotation speed of the grinding table of themill as shown in FIG. 3 to be cyclically and periodically varied.

FIG. 1 shows a greatly simplified schematic representation of a verticalroller mill 10 for comminuting brittle material, e.g. cement rawmaterial. The vertical roller mill 10 comprises a grinding table 12rotatable about the vertical. The grinding table 12 is driven by meansof a heavy duty drive in the form of a motor, in particular an electricmotor 14, and, in the example shown here, by means of a gearbox 16located between electric motor 14 and grinding table 12. The gearbox 16is shown here, without loss of further generality, as bevel-gear teethwith following planetary gearing not shown in greater detail. Thegearbox 16 can also comprise spur-gear teeth or the like and/or apreceding or following planetary gearing or the like.

The vertical roller mill 10 comprises at least one driven shaft. In theillustration in FIG. 1, the vertical roller mill 10 comprises a motorshaft 18 and a grinding table shaft 20. All the means for transmittingthe driving force of the electric motor 14 to the grinding table 12 aretermed the drive train. Here the drive train comprises at least theelectric motor 14, the motor shaft 18, the gearbox and the grindingtable shaft 20.

The vertical roller mill 10 as a whole is a resonant system. Duringoperation of the vertical roller mill 10, the electric motor 14 causesthe grinding table 12 to rotate. On the grinding table 12 there is, as aresult of the grinding process and as a result of supplied material tobe ground, a grinding bed 22, i.e. a mixture of ground material andmaterial to be ground. The grinding effect is achieved by a grindingroller 24 or a plurality of grinding rollers 24 pressing onto thegrinding bed 22 and the rotating grinding table 12 because of theirweight on the one hand, but on the other hand in some cases also becauseof additionally applied forces which are applied e.g. by means of ahydraulic cylinder or the like engaging with a swivel-mounted grindingroller 24.

FIG. 2 shows a simplified schematic plan view of the grinding table 12with the grinding bed 22 and the (here) two grinding rollers 24. Theradial dotted lines within the grinding bed 22 are to indicate anundulation of the grinding bed 22 that frequently arises during thegrinding process. Such undulation of the grinding bed 22 is a possiblecause of the mill rumbling that is to be prevented using the approachpresented here. If the grinding bed 22 is undulating, it is easy to seethat the swivel-mounted grinding rollers 24 follow the surface of thegrinding bed 22 and the thereby caused upward and downward movement ofthe grinding rollers 24 is transmitted to the mill 10 in the form ofvibrations. If the natural frequency of the mill 10 is excited in thisway, resonance can even be set up.

Such vibrations have hitherto been detected by means of a sensor systemdisposed on the mill framework (vibration sensor; not shown). As soon asa vibration measurement acquired by the sensor system exceeds a limitvalue, the electric motor 14 is stopped and the mill 10 is subsequentlyrestarted.

Here it is proposed that a rotation speed of the grinding table 12 iscyclically, in particular periodically, varied. To illustrate this, FIG.3 shows a normally constant rotation speed 26 of the grinding table 12apart from operational fluctuations, and a rotation speed 28 that isperiodically varied according to the approach proposed here, namely arotation speed 28 of the grinding table 12 that is varied symmetricallyabout the original constant rotation speed 26. The frequency underlyingthe periodic variation of the rotation speed 28 of the grinding table 12is in the 0.1 Hz range, for example. The amplitude of the periodicvariation is e.g. in the range of 1% of the effective setpoint speedwithout the variation. In the example shown, the periodic variation ofthe rotation speed of the grinding table 12 has an underlying triangularsignal waveform. Other signal waveforms such as a sinusoidal signal, asquare-wave signal, a ramp-shaped signal, etc. are also possible. Theillustrated periodic variation of the speed 26 is a special form of acyclical variation of the speed. The feature of such a periodicvariation of the speed 26, but also of each more general cyclicalvariation of the speed 26 of the grinding table 12, is that the averagevalue over time of the varied speed 26 results in the setpoint speed andthat the varied speed values continually assume or pass through thesetpoint speed value.

According to the inventor's insight, varying the rotation speed of thegrinding table 12 prevents mill rumbling from occurring at all, becausethe cyclical or periodic variation of the rotation speed of the grindingtable 12 prevents the formation of a regular undulation of the grindingbed 22 as shown in FIG. 2. Varying the rotation speed of the grindingtable, especially periodically varying the rotation speed of thegrinding table, produces a local displacement of the wave contour in thesurface of the grinding bed 22 compared to a state arising in the caseof an unvaried rotation speed. This disrupts the regular excitation ofthe grinding rollers 24, i.e. throws them out of their rhythm somewhat,and no resonance is produced. According to the inventor's insights, aslight variation (ranging from 1 to 5%) is sufficient to achieve thedesired effect. As a result, the mill 10 rumbles less frequently, whichadvantageously impacts availability and the operator's freedoms in termsof process design. In addition, the mill and drive mechanics are subjectto much less stress.

This is a method for drive control of the mill 10 in that, in this way,a surface structure of the grinding bed 22 resulting from the driving ofthe grinding table 12 is reacted to by indirectly or directly varyingthe rotation speed of the grinding table 12 in a cyclical or periodicmanner.

The described cyclical or periodic varying of the rotation speed of thegrinding table 12 can be active continuously or in a time-controlledmanner. In the case of time-controlled activation of the cyclical orperiodic variation of the rotation speed of the grinding table 12 it ispossible for the speed variation to be activated at equidistant pointsin time for a predefined or predefinable time period and thendeactivated again.

The described cyclical or periodic varying of the rotation speed of thegrinding table 12 can also be activated as a function of a statedetected in relation to the mill 10. This can be done using a sensorsystem 30 (FIG. 1) assigned in particular to the drive train, i.e. inparticular the electric motor 14, motor shaft 18, gearbox 16 or grindingtable shaft 20, or to the grinding table 12, to obtainvibration-relevant measured values 32, e.g. torque or rotation speedmeasurements 32. Specifically vibration-relevant measured values 32 inthe form of torque measurements 32 are a measure of the torque orgearbox torque transmitted by means of the gearbox 16, i.e. a measure ofa torque which is termed the mechanically effective torque in the drivetrain, in particular in the gearbox 16, to differentiate it from anelectrical torque acting on the electric motor 14. On the basis of sucha measured value 32 (or the pattern recognition described in theparallel application having the internal reference number 201312099) thecyclical or periodic speed variation can be activated as required, e.g.whenever an acquired measured value 32 exceeds a predefined orpredefinable limit.

To cyclically or periodically vary the rotation speed of the grindingtable 12, a speed variation device 34 (FIG. 1) is provided. Thiscomprises, for example, a signal generator 36 realized in software orhardware and—if the method and its embodiments are implemented insoftware—a computer program 38 and a processing unit 40 in the form orin the manner of a microprocessor provided for executing the computerprogram 38. The computer program 38 is loaded into a memory 42 of thespeed variation device 34.

By generating an appropriate cyclical or periodic signal, the signalgenerator 36 causes the rotation speed of the grinding table 12 to becyclically or periodically varied by e.g. combining a resulting,cyclical or periodic output signal 44 of the speed variation device 34with a speed setpoint value 46 and feeding the combination of the twosignals 44, 46 (addition or subtraction) to a frequency converter 48connected upstream of the electric motor 14 in per se known mannerwhich, on the basis of the input signal 50 thus obtained, produces arespective supply voltage, in particular an AC voltage, for driving theelectric motor 14. The speed of the electric motor 14 thereforefluctuates with the cyclicality or periodicity predefined by the speedvariation device 34 and the resulting rotation speed of the grindingtable 12 also fluctuates accordingly.

Lastly FIG. 4 shows in schematically simplified form that the speedvariation device 34 is e.g. a sub-functionality of a controller 52 orthe like, i.e. an open-loop control device for controlling the frequencyconverter 48. In addition to the speed variation device 34 or a softwareimplementation of the speed variation device 34, the controller 52 canalso comprise other functional units such as a closed-loop controldevice 54 for controlling the speed of the electric motor 14 or similar.The controller 52, the frequency converter 48 and the electric motor 14together constitute a drive system 56 for driving the mill 10. Insteadof the frequency converter 48 or in addition to a frequency converter48, it is also possible for a superposition gear to be used.

In particular embodiments of the approach described here, the signalwaveform underlying the periodic speed variation and/or a period 58(FIG. 4) of the output signal 44 of the speed variation device 34underlying the periodic speed variation, for example, can be selected bymeans of the speed variation device 34.

Although the invention has been illustrated and described in detail byan exemplary embodiment, the invention is not limited by the example(s)disclosed and other variations may be inferred therefrom by the averageperson skilled in the art without departing from the scope of protectionsought to the invention.

Individual prominent aspects of the description submitted here may besummarized as follows: specified are a method for drive control of avertical roller mill 10 having a grinding table 12 rotatable about thevertical, wherein the grinding table 12 can be driven by a drive traincomprising an electric motor 14 and a gearbox 16, wherein a rotationspeed of the grinding table 12 is varied cyclically, in particularperiodically, in order to prevent rumbling of the mill 10, and a drivesystem 56 operating according to the method with which a rotation speedof the grinding table 12 can be periodically varied.

1.-13. (canceled)
 14. A drive control method for a vertical roller millhaving a grinding table rotatable about a vertical axis, the methodcomprising: driving the grinding table by way of a drive train having anelectric motor and a gearbox, and cyclically varying a rotation speed ofthe grinding table.
 15. The method of claim 14, wherein the cyclicalvariation of the rotation speed of the grinding table is activated undertime control for a predefined period of time.
 16. The method of claim15, wherein the rotation speed is activated at predefined points intime.
 17. The method of claim 16, wherein the predefined points in timeare equidistant.
 18. The method of claim 14, wherein the cyclicalvariation of the rotation speed of the grinding table is activated inresponse to a fluctuation of vibration-relevant measured values thatexceeds a predefined limit.
 19. The method of claim 18, wherein thevibration-relevant measured values comprise at least one of a rotationspeed of a rotating component of the vertical roller mill and a drivingtorque and supporting torque operating in or on the gearbox.
 20. Themethod of claim 14, wherein the rotation speed is cyclically varied byperiodically varying of the rotation speed, and wherein at least one ofan amplitude and a period duration of the periodic variation of therotation speed of the grinding table is increased or reduced as afunction of a fluctuation of vibration-relevant measured values thatexceeds a predefined limit.
 21. The method of claim 20, wherein thevibration-relevant measured values comprise at least one of a rotationspeed of a rotating component of the vertical roller mill and a drivingtorque and supporting torque operating in or on the gearbox.
 22. Themethod of claim 14, wherein the rotation speed is cyclically varied byperiodically varying of the rotation speed with a rotation speed profileselected from a sinusoidal profile, a rectangular profile, a square-waveprofile and a ramp-profile.
 23. The method of claim 22, wherein a changein a signal waveform with which the rotation speed of the grinding tableis periodically varied is initiated when a fluctuation ofvibration-relevant measured values exceeds a predefined limit.
 24. Themethod of claim 23, wherein the vibration-relevant measured valuescomprise at least one of a rotation speed of a rotating component of thevertical roller mill and a driving torque and supporting torqueoperating in or on the gearbox.
 25. The method of claim 14, wherein theelectric motor is fed by a frequency converter, and wherein the rotationspeed of the grinding table is cyclically varied by appropriate controlof the frequency converter.
 26. The method of claim 14, whereinvibration-relevant measured values are obtained by measuring with asensor system a rotation speed of a rotating component of the drivetrain or at least one driving torque or supporting torque, or both,operating in or on the gearbox.
 27. A computer program product embodiedon a non-transitory storage medium and comprising a computer programhaving program instructions which, when loaded into a memory of acontrol device controlling a drive for a vertical roller mill having agrinding table driven by at least one electric motor and a drive traincomprising at least one gearbox for rotation about a vertical axis, andexecuted by the control device, causes the control device to cyclicallyvary a rotation speed of the grinding table.
 28. A non-transitorydigital storage medium comprising a computer program having programinstructions which, when loaded into a memory of a programmablecontroller controlling a drive system for a vertical roller mill havinga grinding table driven by at least one electric motor and a drive traincomprising at least one gearbox for rotation about a vertical axis, andexecuted by the programmable controller, cause the programmablecontroller to cyclically vary a rotation speed of the grinding table.29. A drive system for a vertical roller mill having a grinding tablerotatable about a vertical axis, the drive system comprising: anelectric motor driving the grinding table, a frequency converter feedingthe electric motor, a gearbox arranged between the electric motor andthe grinding table, and a rotation speed varying device configured tocyclically vary a rotation speed of the grinding table.
 30. The drivesystem of claim 29, further comprising a sensor system for measuringvibration-relevant measured values.
 31. The drive system of claim 30,wherein the vibration-relevant measured values comprise at least one ofa rotation speed of a rotating component of the vertical roller mill anda driving torque and supporting torque operating in or on the gearbox.32. A controller of a drive system of a vertical roller mill having agrinding table rotatable about a vertical axis, wherein the drive systemcomprises an electric motor driving the grinding table, a frequencyconverter feeding the electric motor, and a gearbox arranged between theelectric motor and the grinding table, the controller comprising aprocessing unit and a memory into which a computer program havingprogram instructions which, when loaded into the memory and executed bythe processing unit, cause the controller during operation of thecontroller to cyclically vary a rotation speed of the grinding table.